Fix AttributeError on SocketModeClient.is_connected (aiohttp) when enabling debug-level logging

[cxong]

In a new aiohttp SocketModeClient, when self.current_session is None, calling is_connected and going into the detailed logging section will attempt to access self.current_session.closed and lead to an AttributeError

Summary

Fix bug where calling is_connected will lead to an AttributeError

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @cxong to sign the Salesforce Inc. Contributor License Agreement.

[seratch]

Thank you! LGTM; once the CI builds pass, we'll merge this PR

[seratch]

It seems that CI builds are pretty slow now and they resulted in timeout errors. I will check the issue later this week

[seratch]

Thanks for the contribution! Your changes have been merged by this commit: 089afec